If I select a group of keys and scale them with a negative scale then all of the keys keep their respective curves but this means that playback is different than if the original animation had been played in reverse, which is unexpected.

Example:
If key A has a linear interpolation to key B and B has a curve interpolation to key C, moving from A to B will have a linear blend but moving from B to A (if you scale it negatively) will have a curved blend.

The curves per key are preserved but the relationship between keys is not, and IMO the relationship between keys is far more important here. This also screws up the timing for any keys that utilizing a non-symmetrical interpolation curve, such as an ease-in or ease-out.

What I would expect to happen when negatively scaling a group of keys is for key N+1 to get the inverse curve of key N so that the relationships are preserved.